(a) Field of the Invention
The present invention relates to a method for polishing the surface of the transparent substrate layer of a color filter unit used in liquid crystal displays, as well as to a pressure-sensitive tape used in the method.
More particularly, the present invention relates to a method for polishing the surface of the transparent substrate layer of a color filter unit used in color televisions, personal computers, etc., which comprises attaching, to the surface of the transparent electrode layer of the color filter filter unit, a particular pressure-sensitive tape capable of preventing the breakage of the color filter unit during polishing and giving, to said surface of the transparent electrode layer of the color filter unit, substantially no staining caused by adhesion of residual pressure-sensitive adhesive, polishing agent, polishing refuse, etc., as well as to a pressure-sensitive tape used in the method.
(b) Description of the Prior Art
In recent years, various flat displays such as LED, PDP, VFD, EL, ECD, flat CRT, LCD and the like have been developed. With respect to color displays, LCD (liquid crystal display) was put into practical application, one step ahead of other displays. LCD can be relatively easily made into a color type by adding a color filter thereto and can be obtained as a thin and lightweight product. Hence, the role of color filter in LCD is important in aiming at the performance improvement of LCD.
In general, soda-lime glass is used as the transparent substrate layer of color filter unit. This soda-lime glass is produced by, for example, the Colburn process or the float process but has surface unevenness.
When a transparent electrode film is formed on such a substrate by vapor deposition, there arises unevenness in film thickness, whereby the non-uniformity of color or the unevenness in threshold voltage appears, making the image clearness strikingly low.
In order to solve these problems, Japanese Patent Application Kokai (Laid-Open) No. 278025/1988, for example, proposes a method for forming a color filter on a glass (transparent substrate) one or both surfaces of which have been polished to a flatness accuracy of 0.15 mm or less.
In this method, however, it is impossible to prevent the reduction in image clearness which is caused by, for example, the scars appearing on the surfaces of the transparent substrate during the transportation, storage, assembling, etc. or the staining of the surface side of said substrate on which no color filter is formed.
Further, Japanese Patent Application Kokai (Laid-Open) No. 116620/1989 proposes a method which comprises forming a color filter layer on a light-transmitting substrate (a transparent substrate), coating a resin thereon to form a resin layer, polishing the resin layer to make it flat, and then forming thereon a tin-containing indium oxide transparent conductive film, i.e. a transparent electrode (ITO).
In this method, however, there are various problems. For example, in polishing the resin layer, scars are formed on the light-transmitting substrate or the color filter layer or there occurs their breakage, in come cases; the polishing refuse, polishing agent, etc. adhere to the color filter layer, increasing the loss in light amount or making non-uniform the spectral transmittance.
In these methods, one or both surfaces of a transparent substrate on which a color filter is formed, is polished to a high flatness accuracy, or a resin layer on which ITO is formed, is made flat; in these methods, therefore, there cannot be expected the prevention of the scars or stains appearing on the surface(s) of the transparent substrate in forming thereon a color filter layer or a transparent electrode layer.
Hence, there was adopted a method which comprises forming, on a transparent substrate, a color filter layer, a protective film layer and a transparent electrode layer in this order to produce a color filter unit, then coating, for a protective purpose, a resist ink, a paraffin or the like on the transparent electrode layer of the color filter unit, followed by drying, and polishing the surface of the transparent substrate of the color filter unit.
This method, however, is not preferable because, after the coating, drying and solidification of resist ink, paraffin or the like and subsequent polishing of the transparent substrate, the resist ink, paraffin or the like must be removed by washing with an organic solvent or the like under heating, which makes the polishing procedure complex, and further the incomplete removal of resist ink, paraffin or the like invites the staining of the surface of the transparent electrode layer.
Thus, it is strongly desired to develop a method for polishing the surface of the transparent substrate layer of a color filter unit, which method is free from the above-mentioned problems.